1. Field of the Invention
The present invention relates to an electron gun for a color cathode ray tube (CRT), and more particularly, to an electron gun for a color CRT which can improve the quality of an image by reducing the diameter of an electron beam.
2. Description of the Related Art
In a general electron gun for an in-line type color CRT, the diameter of an electron beam spot formed on a fluorescent surface is affected mainly by a spherical aberration component of a main lens, an object point component of a triode unit disposed in front of the main lens, and a repulsion effect between electrons in a drift space of an electron beam.
There is an optimal intensity and position of a pre-focus lens which can minimize the diameter of a beam spot corresponding to an aberration component of the main lens of an electron gun. Accordingly, when the aberration component of a main lens is determined, a pre-focus lens having a corresponding intensity and position is formed so that the diameter of a beam spot can be minimized.
The intensity and position of the pre-focus lens varies according to the overall specifications of a CRT, for example, the current of an eletron beam, the distance between the fluorescent surface and the electron gun, and conditions of a first grid facing a cathode of the triode unit. In a state in which the specifications of a CRT is determined, the diameter of a beam spot on the fluorescent surface can be minimized by locating an appropriate optimal point of each component of the triode unit corresponding to the aberration component of the main lens.
In an in-line type electron gun, three electron beams proceed along the same plane. In the case of an in-line type electron gun of a large caliber, in which a main lens has a common area through which three electron beams pass in common and a separate area provided in the common area and each electron beam passes separately, since the common area has a shape of horizontally extending corresponding to the plane that electron beams pass, a spherical aberration component of the main lens in a vertical direction appears greater than that in a horizontal direction. Thus, only when the intensity and position of the pre-focus lens of the triode unit in the vertical and horizontal directions are appropriately and independently adjusted, can a quality beam spot be formed on a screen. For a CRT adopting an in-line type electron gun of a self-convergence type, an electron beam is de-focused in a vertical direction due to an irregular magnetic field generated by a deflection yoke when the electron beam is deflected toward a peripheral portion of a screen so that the beam spot is formed to be elongated vertically. Thus, to solve the above problem, a reduction in the height of the electron beam in the vertical direction, that is, extending the electron beam horizontally, is required.
FIGS. 1 and 2 are sectional views schematically showing a conventional electron gun for a color CRT in which an electron beam controlling means for the horizontal extension of an electron beam is provided. In the drawings, the upper side and the lower side of an Xxe2x80x94X axis indicate structures in a horizontal direction and a vertical direction, respectively.
Referring to FIG. 1, a first electrode G1 a second electrode G2, and a third electrode G3 are disposed at predetermined intervals in front of a cathode K. Electron beam passing holes G11, G21, and G31 through which an electron beam from the cathode K passes are formed in the respective electrodes G1, G2, and G3. A pre-focus lens is formed between the second electrode G2 and the third electrode G3 due to a difference in the electrical potential therebetween. An auxiliary electrode G23 having a slot G22 of an horizontal extension type for forming a strong electric field in a vertical direction than a horizontal direction formed therein is attached to the side surface of the second electrode G2 facing the third electrode G3. Thus, an electron beam B passing through the pre-focus lens is formed to be extending horizontally by the electric field generated by the slot G22 which is stronger in the vertical direction than in the horizontal direction. The conventional electron beam is for adjusting the intensity of the pre-focus lens, but it has a structure which is not appropriate for adjusting the position thereof.
Referring to FIG. 2, in the conventional electron gun, as shown in FIG. 1, the first electrode G1, the second electrode G2, and the third electrode G3 are disposed at predetermined intervals in front of the cathode K. Electron beam passing holes G11, G21, and G31 through which an electron beam from the cathode K passes are formed in the first, second, and third electrodes G1, G2, and G3, respectively. A pre-focus lens is formed in a space between the second electrode G2 and the third electrode G3 due to a difference in the electrical potential therebetween. An auxiliary electrode G24 having a slot G25 of an horizontal extension type for forming a stronger electric field in a vertical direction than a horizontal direction formed therein is attached to the side surface of the second electrode G2 facing the first electrode G1. Thus, a divergent lens formed between the first electrode G1 and the second electrode G2 is formed to be stronger in the horizontal direction than in the vertical direction by the slot G24 and the pre-focus lens between the second electrode G2 and the third electrode G3 is formed to be strong in the vertical direction and relatively weak in the horizontal direction, which has the same effect as the divergent lens making a change in the distance to the pre-focus lens therefrom. The change in position of the lens induces the horizontal extension of an electron beam. The electron gun having the above structure has a structure more suitable for a change in the position than for a change in intensity of the lens.
Both conventional electron guns as shown in FIGS. 1 and 2 are provided with auxiliary electrodes G23 and G24 to the front or rear side of the second electrode which form a stronger electric field in the vertical direction than the horizontal direction so that the electron beam is extended horizontally by each of the slots G22 and G25 of the auxiliary electrodes G23 and G24. Thus, as indicated above, the electron beam extends horizontally while passing through an irregular magnetic field of the deflection yoke and a beam spot which is close to a regular circle is formed on a screen.
However, the conventional electron guns having only a slot have a disadvantage in that it is difficult to induce changes in both intensity and position of the pre-focus lens corresponding to the spherical aberration of the main lens. For example, in the case of the electron gun shown in FIG. 1, the intensity of the pre-focus lens can be adjusted but the position thereof is difficult to adjust, whereas the electron gun shown in FIG. 2 has a structure more suitable for a change in the position than the intensity of the lens.
To solve the above problems, it is an objective of the present invention to provide an electron gun for a color CRT having an improved structure in which the intensity and position of the pre-focus lens can be optimized in each of the vertical and horizontal directions.
It is another objective of the present invention to provide an electron gun for a color CRT which can form a uniform beam spot on the entire surface of a screen so that a quality image can be provided.
Accordingly, to achieve the above objective, there is provided an electron gun for a color cathode ray tube comprising: a triode unit including a plurality of cathodes arrayed in a first direction with a predetermined interval, a first electrode having a plurality of electron beam passing holes corresponding to the respective cathodes, and a second electrode having a plurality of electron beam passing holes corresponding to the electron beam passing holes of the first electrode and maintaining a predetermined distance from the first electrode; a main lens unit for finally focusing and accelerating an electron beam including a third electrode having a plurality of electron beam passing holes corresponding to the electron beam passing holes of the second electrode of the triode unit and forming a pre-focus lens with the second electrode; a first auxiliary electrode having a plurality of slots extended in the first direction or in a second direction perpendicular to the first direction which correspond to the electron beam passing holes of the second electrode and coupled to one side surface of the second electrode facing the first electrode; and a second auxiliary electrode having a plurality of slots extended in the first direction or in a second direction perpendicular to the first direction which correspond to the electron beam passing holes of the second electrode and coupled to the other side surface of the second electrode facing the third electrode.
In the present invention, the slots are beam passing holes through which an electron beam passes and form different electric field in the first direction or the second direction perpendicular to the first direction with respect to the electron beam.